Fuse elements are used in automotive engineering, for example, in order to separate electric power circuits in a defined and quick manner in an emergency. The required standard which such a fuse element has to be up to is that its triggering and its interrupting function has to be reliably guaranteed after up to 20 years even without maintenance. Furthermore, a fuse element may not constitute an additional source of danger as a result of hot gas, particles, objects thrown or high voltages induced in the electric circuit after the latter has been turned off.
One potential field for fuse element application in automotive engineering is the defined irreversible separation of the on-board cabling from the car battery immediately after an accident in order to avoid ignition sources through sparks and plasma, which are produced if cable insulating material was abraded by parts of the car body penetrating the car during the accident, for example, or if loose ends of cables are pressed onto each other or against sheet metal parts and are abraded. If petrol leaks out at the same time in an accident, such ignition sources may ignite ignitable petrol-air-mixtures which collect under the engine hood, for example. Another field of application is the electrical separation of an electrical or electronic component from the on-board supply system in case of a short-circuit in the corresponding component, for example an electrical auxiliary heating.
Pyrotechnic fuses which are actively triggered are known from the prior art. For example, the document DE-AS 2 103 565 describes a current breaker having a metallic housing which is connected at two terminal zones spaced from each other with one end, respectively, of a conductor to be protected by a fuse. In the housing, a pyrotechnic element is provided which is formed by an explosive charge. The explosive charge can be activated by an electrical igniter, which comprises an igniting element which is evaporated by a supply current. The housing is filled with an insulating liquid. The axially extending housing comprises a circumferential groove along which the housing cracks if the explosive charge is ignited. The housing is broken open into two pieces which are electrically separated from each other, so the corresponding electric circuit is separated. In this current breaker, the plasma produced when an electric circuit with a very high current intensity is separated is extinguished by the dispersed insulating liquid. In an automotive vehicle, the fuse may be triggered by the signal of a shock sensor, for example.
A self-ignition for separating the electric circuit in case of overloading of the conductor to be protected by the fuse is not intended in this known device because the entire sleeve would have to be heated up to the ignition temperature and then a detonative combustion or reaction would not be safely achieved since a detonative explosive can hardly be ignited, that is, made to detonate by simple heating of the sleeve. This, however, would be necessary with the type of housing described in the document DE-AS 2 103 565, for example.
In pyrotechnics all over the world, a denotative reaction is said to exist when flame front speeds of more than 2000 m/s are reached.
Another disadvantage of this known device is the problem of permission for devices which contain structural components filled with explosives or even detonators. For this reason, devices of this kind have not been commercially exploited. They are only used sporadically in research institutes for special experiments. Additional reasons for this are the complicated design, the very low handling safety and the extremely high potential of danger which is very difficult to limit.
Furthermore, in many cases, there is a demand for an autoignition function of such a switch or a fuse device in order to protect a cable from overload without having to take the additional effort of providing overload sensors, for example. Thus, a corresponding fuse element should not only be capable of being triggered in a controllable manner, but it should also have the function of a conventional high-current fuse in the form of a safety fuse which can be handled by everyone without danger, as is the case with conventional safety fuses.
High-current safety fuses of this kind have the disadvantage that the turn-off time varies within a large range after the nominal current intensity of the fuse has been reached. Thus, a cable protected by such a fuse can only be loaded to a rather small extent, e.g. 30%, as far as its current carrying capacity is concerned, as otherwise a cable fire might be caused in case of overloading, for example.
From the document DE 197 49 133 A1, an emergency switch for electric circuits is known which is capable of being triggered automatically, but also of being triggered in a controllable manner. For this purpose, an electric conductor is used which has a pyrotechnic core. This core may consist of a propellant charge powder, for example. On the one hand, the pyrotechnic core may be ignited by the heating of the electric conductor when an admissible current intensity (nominal current intensity) is exceeded. On the other hand, it is intended to ignite the pyrotechnic core by means of a controllable ignition device in the form of a heating wire, for example. However, the document DE 197 49 133 A1 merely shows the principle of such a device but does not give any hints on potentially advantageous constructive embodiments. In fact, manufacturing a conductor with a pyrotechnic core of this kind requires considerable efforts. Furthermore, even in case of such an emergency switch, a safe and quick separation of the conductor can only be guaranteed if a detonative explosive is used. If deflagrating substances such as thermite are used, the conductor only bursts open and the residual gas escapes without separating the conductor entirely. The complete separation is only achieved, if at all, by the melting of the conductor as a result of the current flowing through the fuse.
From the document U.S. Pat. No. 3,958,206, a fuse is known in which the current for which the fuse is used is conducted via a fuse element filled with an exothermically reactive material; by activating the exothermically reactive material, the walls of the fuse element burst open and interrupt the current flow. As the exothermically reactive material, PETN is used, for example, that is, a detonatively reacting material, so a fuse of this kind must be up to strict approval standards. The exothermically reactive material may be activated by the dissipated heat of the current itself for which the fuse is used or by an active ignition device. However, if a material burning more slowly was used, for example a so-called propellant charge powder, the housing of the fuse element would only burst open in an undefined and inaccurate manner. Thus, there is a risk that, at the beginning, only cracks or holes are produced in the fuse element and the remaining material of the walls has to be melted by the current for which the fuse is used. This impairs the reaction velocity of the fuse and is not admissible for reasons of reliability, either.
Moreover, the document U.S. Pat. No. 3,958,206 discloses a fuse having a fuse element in the form of a flat conductor, for example, which is coated with an aluminium layer and a palladium layer on top of it. Aluminium and palladium act as exothermically reactive materials; activating the exothermic process may be effected by the dissipated heat of the current for which the fuse is used or by means of an activating device.